The invention relates to a cathode pot of a fused salt reduction cell for the electrolytic production of aluminum comprising a steel shell supported by or reinforced with metal structural parts, a thermally insulating layer and an inner lining which is of carbon, is electrically conductive and resistant to attack by the molten aluminum and the electrolyte.
In the fused salt reduction process for producing aluminum from aluminum oxide the latter is dissolved in a fluoride melt comprised for the greater part of cryolite. The cathodically precipitated aluminum collects under the fluoride melt on the carbon floor of the cell, the surface of the molten aluminum forming the actual cathode. Dipping into the electrolyte from above are anodes which in conventional processes are made of amorphous carbon. At the carbon anodes, as a result of the electrolytic decomposition of the aluminum oxide, oxygen is formed; this oxygen combines with the carbon of the anodes to form CO.sub.2 and CO. The electrolytic process takes place in a temperature range extending from approximately 940.degree. C. to 970.degree. C.
During the operation of the cell, the carbon lining experiences a significant increase in volume. This is caused by the penetration of components from the electrolyte into the carbon lining. By components here is to be understood for example sodium or salts of which the fluoride melt is made up, and also chemical compounds which are formed in the fluoride melt via reactions which are not well understood.
Two other specific, important factors which influence the swelling of the cathode carbon during cell operation are:
The applied current density: the higher the current density, the greater is the increase in volume. PA0 The quality of the carbon: the higher the degree of graphitization, the smaller is the increase in volume.
As it swells, the carbon lining presses against the thermal insulation and thus indirectly on the steel shell. This can result in irreversible deformation which can deform the steel plastically and even result in cracking of the same.
The tendency for the carbon base to swell increases with the age of the cell; cracks form as a result of this swelling. The molten aluminum can penetrate these cracks and attack the iron cathode bars which conduct away the direct electric current. The disruption of the cell lining can progress to such a stage that the liquid aluminum runs out of the cell. In that case the cell must usually be put out of service prematurely. Such an event leads to expensive repairs and of course incurs a loss of production during the down time of the cell.
Numerous attempts have been made in the past to avoid deformation cracks in the carbon floor of the cell by providing reinforcement to the steel shell. As a rule, however, these events could as a result not be prevented but simply lessened. Furthermore, such reinforcement of the cell suffers the disadvantage of adding expense so that the cell becomes more expensive and at the same time the overall weight of the cell is considerably increased.
Other efforts have been aimed at saturating the carbon lining with electrolyte components in advance in order to prevent a subsequent increase in volume. It has been shown however that these volume increases cannot be avoided and must be accepted as unavoidable. Proposed in the U.S. Pat. No. 4,124,476 is to provide a curvature in the steel shell. This comprises a space filled completely first with an easily formable material and secondly with a material which deforms only when large forces are applied to it; the said space accommodates the floor of the carbon lining which expands in the horizontal direction during operation of the cell. The second mentioned material exhibits such properties that the forces in question are transferred to the curved steel shell without permanent deformation and/or crack formation occurring. The counter-forces acting on the floor of the carbon lining diminish its swelling/doming and propagation of cracks through it.
Although the state of the art proposals, in particular those in the above mentioned U.S. Pat. No. 4,124,476, are of some assistance, there still exist great problems with very high current cells.
It is therefore an object of the present invention to develop a new concept for a cathode pot of a fused salt reduction cell for the production of aluminum, such that uncontrolled deformation can be prevented in cells of all size, and such that the cells will not suffer damage in the form of crack formation. The concept is intended to be realized with lower investment costs and to be flexibly applicable.